Magnesium alloys, being one third lighter than an equal volume of aluminum alloys, offer many possibilities for weight reduction, and are, therefore, very attractive in such applications as automotive and aerospace industries. After CAFÉ and other environmental legislation, most car manufacturers have set targets to use 40–100 kg of magnesium alloys per car in the near future. Magnesium alloy components are produced by various casting processes, including high-pressure die-casting, sand casting and permanent mold casting. Other relevant production technologies are squeeze casting, semi-solid casting, thixocasting and thixomolding. According to the forecast of the International Magnesium Association (IMA), the use of die-casting magnesium will continue to grow. An ideal magnesium alloy for making automobile parts, beside being cost effective, should meet several conditions related to its behavior during the casting process and during its use under continued stress. The good castability includes good flow of melted alloy into thin mold sections, low sticking of the melted alloy to the mold, and resistance to oxidation during the casting process. A good alloy should not develop cracks during cooling and solidifying stage of casting. The parts that are cast of the alloy should have high tensile and compressive yield strength, and during their usage they should show a low continued strain under stress at elevated temperatures (creep resistance). The good mechanical properties should be preferably kept even at temperatures higher than 120° C., if the parts are intended as parts of the gear-box or a crankcase. The alloy should also be resistant to the corrosion. The physical and chemical properties of the alloy depend in a substantial way on the presence of other metallic elements, which can form a variety of intermetallic compounds. These intermetallic compounds impede grain sliding under stress at elevated temperatures.
All conventional die casting magnesium alloys are based on Mg—Al system. The alloys of the Mg—Al—Zn system (e.g., commercially available alloy AZ91D) or of Mg—Al—Mn system have good castability, corrosion resistance and combination of ambient strength and ductility, however they exhibit poor creep resistance and elevated-temperature strength. On the other hand, Mg—Al—Si alloys and Mg—Al—RE alloys reveal improved creep resistance but exhibit insufficient corrosion resistance (AS41 and AS21 alloys) and poor castability (AS21 and AE42 alloys). Both types of alloys further exhibit relatively low tensile yield strength at ambient temperature. In addition, high content of rare elements (RE), e.g. 2.4% in AE42, increases the costs. The inclusion of Ca or Sr in the alloy was shown to overcome some of the mentioned drawbacks. German Patent Specification No 847,992 describes magnesium-based alloys, which contain 2 to 10 wt % aluminum, 0 to 4 wt % Zinc, 0.001 to 0.5 wt % manganese, 0.5 to 3 wt % calcium and up to 0.005 wt % beryllium. In addition, these alloys also contain relative high concentration of iron (up to 0.3 wt %) in order to suppress hot cracking problems. The publication GB 2,296,256 discloses a magnesium-based alloy containing up to 2 wt % RE and up to 5.5 wt % Ca. WO 9625529 discloses a magnesium-based alloy containing up to 0.8 wt % calcium which has a creep strain of less than 0.5% under an applied stress of 35 MPa at 150° C. for 200 hours. EP 799901 describes a magnesium-based alloy for semi-solid casting which contains up to 4 wt % calcium and up to 0.15 wt % strontium, wherein the ratio Ca/Al should be less than 0.8. EP 791662 discloses a magnesium-based alloy comprising up to 3 wt % Ca and up to 3 wt % of RE elements, wherein the alloys are die-castable only for certain ratios of the elements. EP 1048743 teaches a method for making a magnesium alloy for casting, comprising Ca up to 3.3% and Sr up to 0.2%. U.S. Pat. No. 6,139,651 discloses a magnesium-based alloy comprising Ca up to 1.2 wt %, Sr up to 0.2 wt %, while Zn is in either of the ranges 0.01 to 1, and 5 to 10 wt %. WO 0144529 describes a magnesium-based casting alloy comprising up to 2.2 wt % Sr.
It is an object of this invention to provide alloys with improved strength at ambient and elevated temperatures, as well as improved creep resistance at elevated temperatures up to at least 150° C.
It is another object of this invention to provide alloys, which are particularly well adapted for high-pressure die casting process, exhibiting reduced susceptibility to die sticking, oxidation, and hot cracking, and which have good fluidity.
It is still another object of this invention to provide magnesium-based alloys suitable for elevated temperature applications, which have a good corrosion resistance.
It is a further object of this invention to provide alloys, which may also be used for other applications such as sand casting, permanent mold casting, squeeze casting, semi-solid casting, thixocasting and thixomolding. It is a still further object of this invention to provide alloys, which can be successfully cast though being beryllium free.
It is also an object of this invention to provide alloys, which exhibit the aforesaid behavior and properties and have a relatively low cost.
Other objects and advantages of present invention will appear as description proceeds.